Plasmodium falciparum, the causative agent of the most severe form of human malaria, rapidly acquires resistance to every new antimalarial agent introduced in the field. The PI wishes to study a new aspect of drug-resistance that could very well be fueling the reemergence of malarial parasites around the world. Preliminary results show that some clones of malarial parasites have a frequency of drug resistance that is about one thousand times higher than some other clones. This phenotype of Accelerated Resistance to Multiple Drugs (ARMD) is different from the well studied Multiple Drug Resistance (MDR) phenotype. ARMD parasites develop resistance to each compound independently. There is no cross-resistance between mutants and the frequency of dual-resistance approaches a multiple of the individual resistance frequencies. To survey the current distribution of ARMD parasites around the world, frequency of drug resistance will be measured in recent isolates of P. falciparum. ARMD phenotype may arise either (i) from increased mutation rates in some clones or (ii) from other preexisting genetic variations. To determine which of these causes underlies the ARMD phenotype: (i) forward rates of resistance will be measured in ARMD parasites using fluctuation analysis, (ii) frequency of drug resistance in ARMD parasites will be measured using a larger battery of novel antimalarial agents, and (iii) the frequency of alteration in nucleotide sequences will be followed on a specific chromosomal site, and on plasmids, in ARMD and non-ARMD parasites. Additionally, non-ARMD parasites will be subjected to a selection protocol for ARMD phenotype and characterized as above. The outcome of these experiments will enhance our understanding of the evolution of drug resistance in malarial parasites and will set the stage for future experiments that define the genetic and biochemical mechanisms responsible for ARMD phenotypes.